Reproductive System Chapter 26 – Day 4 4/23/08

Conception The purpose of reproduction – The sexual act (coitus) allows fertilization of the egg if it (coitus) occurs at the correct time of the ovarian cycle Conception Secondary Oocyte is released – travels slowly down the uterine tube Timing is CRITICAL After release from the ovary the oocyte survives for ONLY 24 hours If sperm is available within this period of time, then fertilization can take place Sperm remains in the female tract in a viable condition for approximately 72 hours 4/23/08

Conception Millions of sperm are released because there are many challenges: Acidic secretions Fluid currents When the sperm reaches the egg Millions crowd the egg As soon as on makes contact… Changes in the egg block the other sperm The acrosome contains hyaluronidase This dissolves the outer wall of the ovum (zona pellucida) The sperm then “enters the egg” – the membranes are fused If fertilization occurs, the second polar body is expelled – the secondary oocyte/egg = ovum (mature gamete) 4/23/08

Conception The membrane of the egg immediately thickens, forming a hard wall (fertilization membrane) This prevents the entry of other sperm An electrical reaction occurs – “cortical reaction” occurs after penetration Enzymes destroy all sperm binding sites on the surface of the egg The chromosome number is now diploid 4/23/08

Fate of the Fertilized Egg At fertilization The haploid nuclei of the ovum & sperm fuse to form a common diploid segmentation nucleus (the ZYGOTE) Occasionally a single fertilized egg splits into two shortly after fertilization = monozygotic/identical twins Sometimes (very rare) the “split” is not complete, so they remain joined = siamese twins If 2 ova are released and each gets fertilized separately by their own separate sperm = dizygotic/fraternal twins These are not genetically identical After the formation of a fused nucleus – DEVELOPMENT BEGINS: Mitotic cell division Cell differentiation/specialization Size increase (growth) 4/23/08

Initial Development Stages Cleavage Multiple mitotic divisions The cell splits into 2 identical cells, these split, yielding 4, etc… Each new cell = blastomere Once there is a solid mass of cells = morula The size of the mass remains the same as the original zygote Fig. 27.2 4/23/08

Initial Development Stages Blastocyst Formation 4-5 days after fertilization There is a rearrangement of the cells Cells arrange along the edge of the morula– forms a hollow ball of cells = blastocyst There are 2 distinct areas The trophoblast = surrounding cells The inner cell mass = cluster of cells at one pole The embryo forms from the inner cell mass The surrounding membrane forms from the trophoblast Fig. 27.2 4/23/08

Implantation The blastocyst enters the uterus (free floating) 8 days after fertilization, the blastocyst becomes attached or implanted onto the uterine wall Orientation of the blastocyst is important The inner cell mass faces the uterine wall It touches the wall Upon contact: The trophoblast separates into 2 layers at the area of contact Fig. 27.3 4/23/08

Implantation The trophoblast has separated into 2 layers: The syncytiotrophoblast = outer layer This layer invades the inner portion of the endometrium Eventually it creates a region where maternal blood will be available for nourishment Enzymes break down the uterine wall The cytotrophoblast = inner layer, surrounding the inner cell mass This layer forms the placenta along with the syncytiotrophoblast Fig. 27.2 4/23/08

Hormone Changes At the onset of implantation Trophoblast layers secrete HCG This maintains the corpus – which maintains levels of estrogen & progesterone Pregnancy tests detect HCG levels Prostaglandins are released by endometrial cells This is triggered by the blastocyst Allows liquefaction of endometrial cells Development up to the 8th week = Embryo Development Development from 8th week to birth = Fetal Development Fig. 27.2 4/23/08

Embryo Development Begins in second week: Gastrulation Inner cell mass forms primary germ layers Ectoderm = outer layer Will become integument, nervous system Endoderm = innermost layer Will become GI tract, respiratory tract & urinary system Mesoderm = middle layer Will become connective tissue, bone, muscle, blood Fig. 27.4 4/23/08

Embryo Development – Weeks 2-5 Extraembryonic layers also form germ layers Yolk sac= endoderm & mesoderm basically a blastocoel layered with mesoderm Blood cell formation Amnion = mesoderm & ectoderm Extension of the ectoderm = a chamber with amniotic fluid Functions to cushion the baby Allantois = endoderm & mesoderm Forms near base of the yolk sac, close to endoderm Extension towards uterine wall Form the umbilical cord & urinary bladder Chorion = mesoderm & trophoblast Chorionic villi with blood vessels Involved in placentation 4/23/08

Embryonic Development Bile capillaries, macrophages Fig. 27.5 4/23/08

Placentation Placenta formation Starts close to the beginning of the 4th week Is completed by the 5th month of pregnancy At the attachment point, the endometrium becomes the decidua (has 3 layers) = (mother/maternal placenta) Parietalis = adjacent to the uterus wall Capsularis = between the embryo & uterus wall Basalis = adjacent to the chorion On the fetal side: Chorion & Chorionic villi Chorionic villi grow into the dicidua basalis, these contain fetal blood vessels originating from the allantois As the embryo & later the fetus enlarges, the chorionic villi, along with blood vessels extend further towards the decidua basalis 4/23/08

Placentation The space between the villi & the decidua basalis Maternal blood vessels release blood into the space, this bathes the chorionic villi This allows nutrients to diffuse in, but no mixing of blood As the fetus extends away, the allantois develops into the umbilical cord with blood vessels leading to the chorionic villi 4/23/08

Chorionic villi and umbilical cord Bile capillaries, macrophages Fig. 27.5 4/23/08

Chorionic villi and umbilical cord Bile capillaries, macrophages Fig. 27.5 4/23/08

Function of the Placenta Diffusion of O2 & nutrients from maternal blood to fetal blood Diffusion of CO2 & wastes from fetal blood to maternal blood Other items that can cross the placenta: Hormones, antibodies, drugs, alcohol Infectious agents: viruses like measles, rubella & aids The placenta releases estrogen & progesterone to maintain uterus wall At birth the placenta detaches from the uterus – it is expelled after birth Umbilical cord is cut – leaves “umbilicus” = belly button 4/23/08

Diagnostics Amniocentesis & Chorionic Sampling Diagnose the “health” of the fetus – protein levels & genetic analysis Done to rule out birth defects – offered only if risk is high Amniocentesis (2nd trimester, 15-18 weeks): Fetus is located by ultrasound Needle is inserted into amnion Fluid is withdrawn Tissue can be cultured, karyotypes, sex determined High levels of protein may indicate spina bifida Chorionic villi sampling (as early as 8 weeks): Catheter is inserted through cervix into uterus Villi are collected for sampling Cells are examined for genetic problems 4/23/08

Back to development While those extraembryonic layers are developing, embryo development is also underway Embryogenesis: Starts at the 2nd week Folding: A head fold & tail fold develop Orientation: dorsal vs. ventral sides become distinct (different) Organogenesis = formation of organs 4/23/08

Bile capillaries, macrophages Table 27-1 4/23/08

Pregnancy Duration = 3 trimesters of 3 months each 1st trimester: Extraembryonic layers are formed & develop as outlined Embryogenesis Organogenesis By the 8th week the fetus is formed All major organs are formed by 3 months 2nd trimester: Fetal growth Grows faster than the placenta Amniotic membrane fuses with the chorion = amniochorionic membrane 3rd trimester: Next slide 4/23/08

Pregnancy Duration = 3 trimesters of 3 months each 3rd trimester: Organ systems develop further towards functioning independently The uterus expands in size In the process it pushes other maternal organs out of the way 4/23/08

Physical Changes in Mother (uterus) Bile capillaries, macrophages Fig. 27.10 4/23/08

Pregnancy - Fetus Full Term In the fetus: Lung, kidneys, GI tract are all NOT functioning Umbilical vein – brings oxygenated blood from placenta to the fetus Blood reaches the fetus – mixed blood pumps through fetus Blood returns to placenta via umbilical arteries Full Term At 9 months – the uterus has expanded extensively from its original size The uterus now contains approximately 5 liters of fluid The average fetus weighs 2.6 kg (7lb) 4/23/08

Physical Changes in Mother (uterus) It is a gradual process that reaches its max. at full term increased smooth muscle contractions in uterus wall Increased stretch The process is controlled by hormone levels HOW does the uterus stretch (look at Fig. 27-10)?!?! Bile capillaries, macrophages Fig. 27.10 4/23/08

Stretching of Uterus - Hormones Implantation: HCG elevates progesterone Progesterone inhibits stretching – OK for initial stages of pregnancy As the placenta develops Estrogen increases Estrogen increases contractions in the uterus As estrogen increases, oxytocin increases in response to estrogen levels Usually at the time of delivery the very high estrogen levels will increase the force and frequency of contractions As estrogen & oxytocin increase, prostaglandins increase in the endometrium = uterus stretches further 4/23/08